Referring to FIG. 1, there is shown a portion of a plasma chemical vapor deposition (PCVD) lathe. The PCVD lathe includes a gas supply unit 10 and a substrate tube 12. Gas supply unit 10 includes input ports 14, gauge ports 16a and gauges 16b, a rotary union 18, a bellows 20, and a torr seal extender 22. Gases (e.g., SiCl4, O2, GeCl4, C2F6, etc.) are inputted into gas supply unit 10 via input ports 14. As the inputted gases travel toward substrate tube 12, gauges 16 are configured to monitor and maintain a steady gas pressure within gas supply unit 10 and substrate tube 12.
Rotary union 18 is configured to rotate substrate tube 12 about a longitudinal axis as is indicated by arrow “A.” Torr seal extender 22 is a tube of inert material that is connected to substrate tube 12. Bellow 20 is disposed between rotary union 18 and torr seal extender 22, and is configured to force the gas mixture through torr seal extender 22 to substrate tube 12. Although in FIG. 1, a gap is shown between each of gauges 16 and rotary union 18, rotary union 18 and bellows 20, and bellows 20 and torr seal extender 22, each of these gaps are preferably sealed with clamps, o-rings, flanges, and/or caps such that gas supply unit 10 is maintained at a controlled (e.g., low pressure) environment.
The gas mixture is provided to an internal region of substrate tube 12, transformed into reactive species by plasma using a microwave applicator 15, and is used to coat or deposit one or more layers onto the internal region of substrate tube 12. Any waste gases are removed by an exhaust pumping unit 17. Such coated substrate tube 12 is subsequently collapsed, for example, by an oxy-hydrogen flame, as is well known in the art, to a fiber preform or collapsed cane.
The outer diameter of the preforms or canes formed by PCVD can vary as a function of the axial position or direction. For example, the outer diameter variation can be cyclic or periodic. FIG. 2 shows an example of the periodic variation in the outer diameter of a collapsed cane formed using PCVD. A vertical axis 30 indicates the outer diameters of the collapsed cane and a horizontal axis 32 indicates the axial positions of the collapsed cane. A line 34 fitted to the data shows the variation in the outer diameter to be substantially sinusoidal as a function of the axial position. The periodicity of the variation is 6.537 cm with respect to the axial direction. The difference between the minimum and maximum values of the outer diameter is 0.2126 mm. This difference translates to a mass difference of 0.12 g/cm of the collapsed cane. In other words, the amount of material deposited or coated onto the internal region of substrate tube 12 varies along the axial direction by an amount of ±4.3% by weight. Bending sensitivity due to cut-off variations and/or signal dispersion can be associated with an optical fiber formed from such a collapsed cane because the optical fiber will retain at least some of the non-uniformity of the cane when subsequently drawn.
Thus, there is a need for an apparatus and method of uniformly coating an internal region of a substrate tube along the axial direction. Further, there is a need for an apparatus and method of forming a collapsed cane having very small diameter variations along the axial direction.